Changing strategy is far better than sticking with a back three and always losing, and it would be a good thing if Amorim seriously considered his new formation. He is indeed a bit stubborn, as in several interviews he has consistently stated he won't change his formation. Conceding goals is nothing more than the defense's difficulty in blocking the opponent's attacks, and that is indeed Man United's weakness so far, which Amorim needs to address immediately.
Sunderland are not a tough opponent and they are motivated after achieving promotion to the Premier League and teams like this usually have no burden in playing. So if Amorim still maintains his ego by using three defenders, it will not be surprising if Man United later suffers another defeat.
Amorin should try something new about the formation and change it for the good of his team, because in controlling the ball the defensive midfielder is considered less solid and that is what needs to be fixed now when I pay attention to his shortcomings. Even though Amorin has become accustomed to his three central defenders with the formation of the number of defenders being four, it will build a more stable strength when the opponent counterattacks.